Orthogonal Frequency Division Multiplexing, OFDM, has been selected in many communication systems, e.g. in 3rd Generation Partnership Project Evolved UMTS Terrestrial Radio Access, 3GPP E-UTRA, and Digital Subscriber Line, DSL, systems, such as Asymmetric Digital Subscriber Line, ADSL, systems. Also, OFDM is used for ordinary broadcasting systems, such as Digital Audio Broadcasting, DAB, and Digital Video Broadcasting, DVB, systems. Thus, OFDM is used for both wireless and wireline systems carrying data on a large number of subcarriers. These systems can facilitate high spectral efficiency, since OFDM is suitable for, for example, combination with Multiple Input Multiple Output processing (MIMO-processing) and/or opportunistic transmission schemes.
However, the spectral efficiency of the systems, for both wireless and wireline systems, also depends on the level of the out-of-band power emission, i.e. the power level of the OFDM signal being emitted outside a designated transmission bandwidth. If the out-of-band power is efficiently suppressed, adjacent frequency channels can be spaced densely, and thereby spectrum utilization is improved in the system. Also, the out-of-band emissions must be kept below certain levels in order not to cause significant interference in adjacent frequency bands.
For these reasons, in many system standards, the out-of-band power emissions are regulated and restricted. Several types of requirements exist to regulate the out-of-band power emissions of a signal. In E-UTRA for example, spectral masks, adjacent-channel-leakage-ratios and occupied bandwidth requirements have been defined.
An OFDM signal (being a multiplex of a large number of finite-length complex-valued exponential waveforms) has a power spectrum determined by a number of squared-sin c-shaped functions, where sin c(x)=sin(x)/x. Typically, due to the finite-duration of the exponentials, OFDM signals will not meet requirements on out-of-band emission in a standard, since the spectrum side lobes decay slowly. This slow decay causes the OFDM power spectrum to become relatively broad, resulting in problematic out-of-band emissions, which have to be reduced in some way.
The power spectrum of an OFDM signal is determined by two quantities; a pulse shape and a correlation between the transmitted symbols. When all data symbols in the OFDM signal are uncorrelated, the slow decay of the OFDM spectrum is caused by the finite-duration property of the pulse shape for the individual OFDM symbols constituting the OFDM signal.
In the prior art, two categories of methods for reducing out-of-band emission have been developed, where each of these two categories deals with either the pulse shape or the correlation between the transmitted symbols.
In prior art, time-windowing of the OFDM signal has been proposed in order to tie consecutive OFDM symbols together. This method belongs to the first category mentioned above, i.e. it changes the pulse shape, and uses a prolonged cyclic prefix and an additional postfix. A time-windowed postfix of a previous symbol overlaps with a time-windowed cyclic prefix of a current symbol. However, due to the use of a longer cyclic prefix used by the method, the symbol rate and/or spectral efficiency of the system decrease when the method is implemented. Alternatively, instead of prolonging the cyclic prefix, the overlap could be extended into the cyclic prefix of a succeeding OFDM symbol. However, this would cause intersymbol interference, and would hence reduce an effective length of the cyclic prefix, which would result in a higher sensitivity to channel dispersion.
Also, time-windowing could be performed without overlapping the two consecutive OFDM symbols. This variant can be regarded as a ramping in the front and end of the OFDM symbol, forcing its beginning and end to zero. However, the ramping method results in a shorter effective cyclic prefix, and thus also in a higher sensitivity to channel dispersion.
Further, lowpass transmit filtering of the OFDM signal, in order to shape the power spectrum, which is used in some prior art solutions, also results in intersymbol interference and a reduced effective length of the cyclic prefix, and hence also in higher sensitivity to channel dispersion.
Moreover, in some prior art solutions belonging to the second category mentioned above, i.e. introducing correlation between transmitted data symbols, data subcarriers are pre-processed prior to the IFFT. According to one method, data symbols are weighted with real-valued numbers. These weights are chosen to reduce the out-of-band emissions caused by the rectangular pulse shape. Due to this weighting, the Bit Error Rate, BER, will increase the more the out-of-band emission is suppressed.
Further, in other prior art solutions belonging to the second category mentioned above, i.e. introducing correlation between transmitted data symbols, cognitive multi-band OFDM systems have been considered where the problem is to achieve low interference in certain parts of the frequency band. Methods have been proposed, whose purpose is to create frequency notches within the OFDM frequency band, in which other systems could transmit.
These solutions achieve a form of an in-band power emission reduction, where the interference in a so called victim-band should be minimized. The unwanted power in the victim-band is due to the finite duration of the OFDM symbols, which translates to an unlimited width of the frequency spectrum, such that the frequency spectrum for OFDM only is zero at the subcarrier frequencies. Hence, there will be undesired in-band power at frequencies located between subcarriers.
Prior art methods compute the interference contribution of the data symbols to the victim-band at target frequencies located between the subcarrier tones of the system. Instead of creating a guard band by nulling subcarriers next to the victim band, these subcarriers, and possibly also subcarriers within the victim-band, are modulated so as to minimize the power in the victim-band. This concept has been referred to as Active Interference Cancellation, AIC. The AIC can be posed as a least-squares problem defined in the frequency domain, where the solution is the modulation symbols that should be used on the reserved AIC subcarriers in order to cancel as much as possible of the interference power within the victim-band.
Further, in other prior art solutions belonging to the second category mentioned above, i.e. introducing correlation between transmitted data symbols within one OFDM symbol, the usage of modulated cancellation subcarriers for out-of-band emission has been described. The general idea of these solutions was the same as for the AIC concept, but here the victim band was a side-band (and not an in-band as in AIC). However, the performance of these methods is poor.
The methods belonging to the second category described above modulate cancellation subcarriers in order to reduce unwanted emission, and include design parameters that must be specified, for example the target frequencies that should be suppressed. Further, the principle of these methods is to try to compensate for unwanted out-of-band, or in-band emissions, at certain target frequencies, which in the methods have already been accepted as being present. Thus, these prior art methods do not try to directly correct the fundamental cause of the unwanted emissions, i.e. the finite duration of the OFDM symbol. Instead, they try to reduce the negative effects resulting from these emissions.
Thus, the prior art presents inefficient solutions having drawbacks regarding symbol rate, spectral efficiency and interference.